1. Field of the Invention
This invention relates to a porous ceramic film which has a high porosity, a good pressure resistance and a regulated pore size, and a process for producing the same.
2. Description of the Prior Art
In recent years, there is a growing necessity for filters and catalyst supports having a good heat resistance, a high strength and a high thermal shock resistance. It is believed, for example, a filter or a catalyst support for eliminating CO.sub.2, NO.sub.x or black smoke from an automobile exhaust gas should withstand high temperatures exceeding 1,000.degree. C. Similarly, a high thermal resistance is required of a filter for desulfurizing an exhaust gas from a thermal power plant or a chemical plant, a filter for eliminating slugs from a molten metal, etc.
Attempts have been made to apply porous ceramic films to these filters and catalyst supports and some of them have been already put into practical use. It is expected that silicon nitride, which is excellent in strength, toughness, thermal shock resistance, chemical resistance, etc., and thus has been employed as a structural ceramic material, might be useful as a material for producing porous ceramic films.
Attempts have been also made to employ porous ceramic filters in the fields of, for example, food and drug. Namely, ceramic films have been taking the place of porous organic films which have been employed in these fields, since ceramic films are superior to the organic ones in thermal resistance, pressure resistance, chemical resistance and separative power.
Moreover, porous ceramic films have been used as catalyst supports, bioreactors and supports for microbial incubation. In these cases, it is known that the efficiency of a catalytic reaction or the efficiency of microbial incubation is elevated with an increase in the specific surface area (i.e., surface area per unit weight) of a porous ceramic film.
In order to use a porous ceramic film as the filter or catalyst support described above, it is important for the film to have such a high porosity as to suffer from little pressure loss during filtration and a regulated pore size. When used as a bioreactor, etc., a porous ceramic film is required to have a large specific surface area in addition to the abovementioned requirements.
Furthermore, the permeability, which is the most important factor of a filter, is determined fundamentally depending on the pore size and the porosity, as shown in the Hagen-Poiseuille equation of the following equation 1. That is to say, permeation flow rate at filtration is increased, or the pressure loss is decreased, with an increase in the pore size and a decrease in the thickness. EQU dQ/dt=n.pi.r.sup.4 .DELTA.P/.8.eta.l Equation 1!
wherein
n stands for the number of pores; PA1 r stands for the radius of pore; PA1 .DELTA.P stands for the differential pressure; PA1 .eta. stands for the fluid viscosity; and PA1 1 stands for the thickness.
Meanwhile commercially available conventional porous ceramic filters are composed of a film part having a small pore size for practicing filtration and a base part having a large pore size for supporting the film. Such porous ceramic filters are produced by sintering powdery materials. Therefore, the pore size thereof can be regulated to a certain extent. For example, for such porous ceramic filters, there have been produced porous ceramic bodies having a fine pore size of 5 .mu.m to 0.004 .mu.m (Nihon Gaishi K. K.'s catalog) and, especially, ones having a pore size not greater than 1 .mu.m have been extensively used. However, the maximum porosity of the film part is limited to at most about 40% by volume.
In the base part having a large pore size, the low porosity causes no pressure loss during filtration. In the film part having a small pore size, however, the low porosity results in a serious pressure loss. Moreover, the low porosity brings about another problem that the filtered matters can be hardly removed by loading back pressure thereonto, which deteriorates the regenerative characteristics of the filter.
In addition to the above-mentioned porous ceramic filters, porous glasses, etc., have been used as porous ceramic films for bioreactors (porous glass "VYCOR" mfd. by Corning Glass Works). However, these porous glasses have the problem of a low reaction efficiency because of the further lower porosity of from 20 to 30% by volume and the small specific surface area of about 200 m.sup.2 /g. In addition, a porous glass, which can be hardly processed into a thin film, has a very poor permeability.
Accordingly, there arises an idea of producing a porous ceramic film with a high porosity by depositing ceramic fibers or whiskers in the form of a film onto a base. Although a high porosity can be achieved by this method, it is highly difficult to give a pore size of 0.1 .mu.m or less thereby. This is because the pore size is determined substantially depending on the diameter of fibers or whiskers and it is very difficult to produce fibers or whiskers of a diameter of 0.1 .mu.m or less (Cer. Mat. Compon. Engines (1986), pages 101-108)
In the case of fibers having a large aspect ratio (i.e., the ratio of length to diameter), in particular, the three-dimensionally intertwined structure of the fibers results in an extremely large pore size as compared with the case of whiskers. When the aspect ratio exceeds 200, for example, it is needed to regulate the fiber diameter to 0.01 .mu.m or less in order to give a pore size of 0.1 .mu.m. However, it is impossible to produce fibers having such a small diameter.
Although fibers or whiskers once formed may be mechanically shaped (Japanese Patent Laid-Open No. 3-150275), it is obviously impossible to finely and accurately control the pore size by mechanically pressing an aggregate of fine fibers or whiskers. It is also impossible to process these fibers or whiskers into a film. Accordingly, such a mechanical means is an altogether inappropriate process for producing filters, etc.
When fibers or whiskers are merely deposited in the form of a film onto a base, the three-dimensionally intertwined structure per se has only a weak binding force and a poor adhesiveness to the base. Thus it cannot be used as a filter, etc., because of the insufficient mechanical strength and pressure resistance. Thus Japanese Patent Publication No. 31174/1994 has proposed a method for achieving a large porosity and an improved pressure resistance by depositing whiskers onto pores inside a porous ceramic. However, this method fails to sufficiently improve the pressure resistance and, furthermore, cannot be applied to thin-film filters, although this idea can be applied to bulk porous bodies.
On the other hand, the specific surface area of a fiber or a whisker is determined substantially depending on the diameter and length thereof. Supposing that a fiber or a whisker is in the form of a column of R in radius and L in length, the specific surface area S of an aggregate of a density D is determined by the following equation 2. EQU S=2(R+L)/DRL Equation 2!
Calculation in accordance with the above equation indicates that when the diameter of a fiber or a whisker is 0.1 .mu.m, then the specific surface area is less than 15 m.sup.2 /g, i.e., smaller than that of a porous glass. Accordingly, the high porosity of fibers or whiskers cannot be fully utilized merely by depositing the fibers or whiskers onto a base and thus the obtained film is unsuitable for a bioreactor.